(1) Field of the Invention
The present invention relates to a multipoint data packet switching communication system and an apparatus for extending the communication coverage of a broadcast channel by means of coupling separate such channels together.
(2) Prior Art
Packet switching networks came into existence in response to the need for efficient and economical data communications. As networks grew, there has been a need for inexpensive, high-capacity communication channels. Satellite communication technology may offer a solution with its broadband channels. The satellite acts as a pure transponder repeating whatever it receives and beaming this transmission back down to earth. An alternative method of providing communications among a large number of data communication devices is through a ground radio packet switching system. Ground radio systems have been available for many years. The basic principles involved are similar to the satellite packet switching system. In both systems there is a broadcast channel which is shared among a number of distributed users.
To expand the signal coverage of a broadcast channel, more sophisticated transceivers or packet repeaters may be employed. There is a trade off between using such devices. Transceivers with increased power and sensitivity become more expensive and less reliable. The introduction of repeaters into a broadcast channel makes the interconnecting medium active. Hence, the failure of a repeater partitions the broadcast channel and degrades the communications.
A broadcast channel can support a limited number of transceiver devices. At some point the shared broadcast channel will be so busy that additional stations will just divide up more of an already inadequate bandwidth. To reduce the traffic, a broadcast channel may be split into separate segments. An unbuffered repeater with an address filter may be used to couple broadcast segments together. Such a device would pass packets from one broadcast segment to another only if the destination station is located on the new segment. Only one repeater can connect two separate broadcast segments together since a packet repeated multiple times on a segment by multiple repeaters would cause interference. Hence, a failure of a single repeater connecting two segments would partition the network.
The interconnection of broadcast segments could also be done with packet gateways and the software addressing convention they implement. A gateway approach for coupling point-to-point systems together is described by Cerf, V.G. and Kahn, R.E. in "A Protocol for Packet Network Intercommunication," IEEE Trans. Comm. May 1974, Pages 637-648. A gateway only repeats packets addressed to itself as an intermediary. Since gateways have packet buffers, a number of gateways may be used to connect two broadcast segments. Hence, a failure of a gateway need not partition the network if there are paths through other gateways between the segments.
An address is used to identify destination stations within a network. An address is a data structure whose format can be recognized by all elements in a system and defines the fundamental addressable object. An address must be meaningful throughout the total system and must be drawn from some uniform address space. This address space may be a "flat" one which spans the entire domain such as social security numbers or it may be a hierarchical address space such as telephone numbers with area codes. At the time a device wishes to communicate with a particular address, there will be some mechanism within the network which will map an address into an appropriate route.
In the case of broadcast transmission, which is on a local segment, all messages take the same route, so that only the address is needed to transmit the message. However, in the case of an intersegment message, a routing system is needed since each segment is connected to only a subset of the other segments. Therefore intersegment messages will take different routes to different designations.
The format of local and intersegment addresses in the packet header should be compatible so that a segment interface can recognize an intersegment packet from among those being broadcast locally. The segment interface which handles the packets at the destination segment should be able to use the local broadcast mechanism to send information to the destination station. The address mechanism should serve to transmit both local and intersegment packets. It should also be simple enough to be implemented in low cost dedicated hardware. This would avoid having to install complex routing algorithms or elaborate routing tables such as those implemented in conventional gateways. These are some of the problems this invention overcomes.